Ink jet printing mechanisms use a print head (also call a xe2x80x9cpenxe2x80x9d) that shoots droplets of ink or colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including printers, plotters, copiers, and facsimile machines. As used herein, and in the appended claims the term xe2x80x9cprinterxe2x80x9d or xe2x80x9cprinting devicexe2x80x9d will be understood to refer to all devices that incorporate an ink jet printing mechanism and output a hardcopy product.
An ink jet printer typically includes a print head having a multitude of independently addressable firing units. Each firing unit includes an ink chamber connected to a common ink source, and an ink outlet nozzle. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles. In thermal ink jet printers, the transducers are firing resistors that generate sufficient heat during application of a brief voltage pulse to vaporize a quantity of ink sufficient to expel a liquid droplet from the outlet nozzle.
The energy applied to a firing resistor affects performance, durability, and efficiency. When sufficient firing energy is applied, a vapor bubble forms or nucleates to expel an ink drop. Above this threshold, in a transitional range, increasing the firing energy increases the drop volume expelled. Above a higher threshold, at the upper limit of the transitional range, drop volumes are substantially constant with increasing firing energy. It is in this range, in which drop volumes are stable even with moderate energy variations, that printing ideally takes place. This is because variations in drop volume cause inconsistencies in the printed output and thereby degrade print quality. As energy levels increase above this optimal zone, uniformity is not compromised, but energy is wasted. Moreover, the printer components are prematurely aged due to excessive heating and ink residue build up.
In general, the power or current required for each print job is unknown and depends upon the characteristics of the print job. Print jobs may vary from a relatively small amount of print data, where the print head will not need to draw much current to execute the print job, to a large amount of print data which may require the print head to draw large amounts of current. Additionally, there may be relatively sudden shifts in the amount of data being printed as the print head moves from a relatively blank part of the print job to a more densely printed area. A step load is a substantially instantaneous and significant increase in the amount of current being drawn by the print head due to the demands of the data being printed.
In order to reduce the likelihood of the overall voltage dropping below the threshold needed to properly operate the ink jet pens, the ink jet pens could be operated at a level above the minimum threshold. Operating at this increased voltage level compensates for the potential drop in voltage attributable to a step load or similar event, which is the worst-case scenario. However, this constant, unnecessarily high voltage wastes energy and increases print head temperature thereby reducing the life of the print head.
In one of many possible embodiments, the present invention may be a method of providing power to a print head by determining a printing density at times throughout a print job based on data representing the print job, transmitting a measurement of the printing density to a printer and controlling the power applied to the print head in accordance with the measurement of the printing density.
In another possible embodiment, the present invention may be a printer that can anticipate the power needs of a print head. The printer receives a print job and a measurement of print density as a function of time for the print job from a printer driver that determines print density as a function of time based on data representing the print job. The printer includes a processor for receiving a print job, a print head and a power system for supplying power to said print head. The processor controls a voltage applied by the power system to the print head in accordance with the measurement of print density.